A "Mutant Adventuring" supplement for Fourth Edition D&D that tries
to emulate Gamma World while staying compatible and balanced with
core D&D4. It introduces new races, classes, powers, equipment,
and creatures/NPCs along with a few rules changes such as modified
skills.
347 pages rules (PDF).

A non-traditional fantasy setting for use with any rule system.
Gehennum is a volcanic and mountainous archipelago ringed with
coral and clad in jungle. It is closer to south-east Asia
rather than medieval Europe in nature and culture.
~40 pages HTML.

A detailed adaptation of "Highlander" for the Storyteller
system. It includes character creation mechanics and a variant
combat system, but is not a complete standalone system.
54 pages (PDF, HTML, or text-only).

A very detailed fantasy world with non-system-specific notes. It
is intended to be realistic with fantasy additions. There are
traditional dwarves and elves along with Avarians, Saurians,
and other races. It details 35 nations and 26 cities along with
races, fauna, flora, and other information.
~250 pages HTML.

This is a free live-action conversion of the Silhouette game
system (used in "Gear Krieg", "Tribe 8", and other games by Dream
Pod 9). It requires one of the main rulebooks of those systems
for character creation and other aspects of play.
16 pages PDF.

This is a universal system adapted from the out-of-print TSR games
"Dragonlance: Fifth Age" and "Marvel Superheroes Adventure Game".
The rules are complete but use a special Fate Deck for action
resolution (which can come from either of the games). Players
have a hand of cards, which they play from for each action:
attribute (1-9) + skill (0 or 4) + card total vs difficulty.
The suit of cards and the PC's Edge may allow more than one
card to be played.
~10 pages (HTML or MSWord).

A former free RPG, now a cheap supplement for the Sorcerer
RPG. It is a surreal modern conspiracy/horror setting, inspired
mainly by the films of director David Cronenberg. The PC's are
secret revolutionaries with psychic powers, in the near future
fighting against a corrupt society.
36 pages background (PDF).

This is a preview version of the Traveller20 game, which is
an adaptation of the original Traveller RPG (from Far Future
Enterprises) to the "D20" system from D&D third edition.
The lite rules include complete listings for all skills and
feats. However, it refers to the D&D Players handbook
for basic combat rules, which it modifies extensively. It also
includes only the Merchant class, as opposed to 16 classes in
the non-free game.
64 pages (PDF).

A worldbook for the free CARE system, for
a magical version of modern-day earth, where five tribes (humans,
were-creatures, magical "shapers", undead, and gods) live together
in the 20th century.
~20 pages rules and background.

A sourcebook for the HERO system based on a post-apocalyptic
future featuring rules and background. It includes character
creation with random mutation tables, various technology
notes, and background.
~80 pages (HTML).